Many people experience difficulties with their vision. The most common ametropic (refraction error) vision problem is a condition known as myopia or nearsightedness. Myopia is a common condition in which an eye cannot focus on far-away objects because the cornea of the eye is curved too steeply (i.e., the radius of curvature of the cornea is smaller than normal) to provide adequate focusing at the retina of the eye. When the curvature of a cornea is too flat, the eye cannot focus on near objects and the result is hyperopia (farsightedness). Hyperopia is common among young children, and severe hyperopia will induce lazy eye or amblyopia in childhood. Another form of ametropia is astigmatism, in which unequal curvature of one or more refractive surfaces of the cornea prevents light rays from focusing clearly at one point on the retina, resulting in blurred vision.
Myopia, hyperopia, and astigmatism are conditions for which no entirely suitable permanent treatment has been developed. One approach to correcting ametropia is through surgical reshaping of the cornea. However, such surgical procedures have not proven to be entirely safe, and there is some question as to the permanency of the surgically altered corneal shape.
Another approach to treating some or all of these conditions is to alter the corneal shape by wearing hard contact lenses which are designed to continually exert pressure on selected locations of the cornea to gradually force or mold the cornea into the desired corneal curvature. A retainer lens is then worn on a part-time basis to prevent the cornea from returning to its previous shape. This method of treatment is commonly referred to as orthokeratology (also referred to as “ortho-k”). The success of any treatment by orthokeratology is dependent upon the shape and structure of the contact lens.
The overwhelming majority of ortho-k contact lenses are made of rigid gas permeable materials. The rigid materials used in ortho-k, however, can cause discomfort. Efforts to relieve such discomfort have included the addition of soft lens material to the periphery of a lens having a rigid center, resulting in what have been called hybrid lenses. The cost and durability of such lenses, however, have been a concern to practitioners. Some researchers have also tried to reshape the cornea by using an inverted (inside out) soft contact lens, hoping that the inverted front curvature might flatten the cornea for myopia reduction. The results of such use, however, have been unpredictable.
U.S. Pat. No. 5,963,297 to Reim and several patents to Stoyan (U.S. Pat. Nos. 5,349,395; 4,952,045; 5,191,365; and 6,010,219) disclose ortho-k lens designs for myopia reduction. These patents teach lenses having particular lens curvatures for performing orthokeratology.